During the heavy March rains and melt in Toronto, my basement floor was wet. Lifting/removing the carpet and underlay revealed a layer of what I think is a thinset mortar improperly laid in an attempt to create a level floor. After a few days of running air movers the thinset has pretty much completely delaminated from the original slab, and can now be peeled away using a flatbar (heck, even my hands). I understand enough to know that without first abrading then applying a bonding agent, any type of concrete product will not adhere to an existing concrete slab.

As I was removing the thinnest, I was surprised (and confused) to find a regular series of ½" diameter holes drilled through the floor. These holes were bored after the the thinset was poured, as I can see that the underlay adhesive was acting as skim over the hole(s). The holes are exactly 8" from the perimeter of the foundation walls, and vary randomly from 12-24" on-center. It's almost like someone snapped chalk lines parallel to the foundation, then walked along with with a hammer drill and a ½" masonry bit. The holes are completely bored through the the slab (wire coat-hanger probes measure a 3-4" slab depth). I can shove the wire further in the soil below. Now that the slab is revealed, I find a decent concrete floor (sloped towards the floor drain (which was hidden under vinyl-roll-type flooring)): This slab was not floated smooth, but has generally decent slope, no cracks (happy surprise), and no signs of spalling or staining.

My Questions:
1-Why would someone drill a series holes through a slab into the ground below? My guess is that a previous owner (we've owned the place for 3 years) must have experienced a flood and drilled the holes as 'indoor weepers'. My problem with this is the regularity with which the holes were drilled - If the water table rises, you're just going to get more water back into the basement. I don't think they would have been for stud-wall anchors (8" offset for a 13-foot wide house would really reduce floor space, and you'd think they would not drill all the way through the slab).

2-Would it be appropriate to fill these holes with hydraulic cement compound? I'm thinking of either a King swift-set or plug-tite product.

Any suggestions as to why the holes are there, and how (or not) to plug them is greatly appreciated. Thank you.

Vincer

04-18-2011 02:01 PM

Termite control? We had termites (both types! Subteranean AND drywood!) To defeat the subteraneans, 1/2 inch holes were drilled all along the EXTERIOR of the house in very regular intervals. Termite poison (Premise 75 in our case, but Termidor is likely as well) was injected into each hole. However, after doing it, the termite guys then hammered 1/2" plastic plugs into the holes and covered them with some concrete patch. There are some holes inside the house as well where we have sleeper floors in the "basement" (ie finished crawlspace).

Termites are a problem everywhere (though I also lived in Mississauga for most of my life I don't recall it being an issue for my parent's house, though at Christmas I mentioned termites to a friend in Oakville and he said he cut down a tree FULL of them). I'm in California now where every other house is infested with the buggers!

I don't know why you would want the holes in your slab, now that you have discovered them. If I were you, I would check to see if maybe it was termites. If it WAS, then maybe go ahead and inject MORE poison (it only lasts 5 years) into the holes (get a pro to do it) then get some plastic plugs, hammer them in below the surface and patch it up. If it was for termites and the poison is now gone, you now have an excellent path for termites to come back up through the holes!

It could also be that the previous owner had some unexecuted master plan to use the holes as keys for more concrete or something (like stick rebar in there and add another layer of concrete)

I googled "Toronto Subterannean Termite" and it's apparently a problem there, so I'm 90% sure that's what your holes are, but ... who knows. Do some research on the topic and see if you come to the same conclusion. Good luck! (Check also for old termite damage to see if there is any, if there is then that might add weight to the termite theory). If that's what you think it might be, then it really would be cheaper to get a pest guy in there to inject non-repellant termiticide in those holes. 75% of the labour cost would be drilling the holes, so it's already done for you, the company might give you a break on price since it's just injection. OR do it yourself, but be careful.

Vince

BalowskiJ

04-18-2011 02:33 PM

Vince,

Thank you and spot on! When we got closed on the place it came with a termite-free certification from a local company (as an update from your Mississauga days - termites have made it to Toronto, and a quite a pest in the east end/beaches (where we are)). Since the house is a semi-detached (with 24" separation between the neighouring property) the exterminators treat for subterranean termites from the inside, boring holes, well, exactly like a described. Just found it on their site. Now I can contact them to see if I can't leverage a freebie or discount or the re-treatment since they didn't plug/cement the holes (again, like promised on their site). You've solved a mystery and given me chance to add extra protection against pests before I seal (or they seal) the holes, and that is a real kindness!

Your guidance is greatly appreciated! Thank you again.

Vincer

04-18-2011 02:49 PM

Hey, great!

It was a lucky guess because we just had our house done 3 weeks ago and I was really thinking "Hey, I know how to plug 1/2" holes because those guys just did it when they were ... hmmmm... walking around the house with a hammer drill ... click!"

Anyway, I'm glad I could help, it's not often that I say something useful on a forum! Good luck!